For reading, writing and much more, I could recommend the MetaLib from www.trading-tools.com With 49$ (if I remember right) it's price is quite attractive and it's real EASY to use in C#, VB, Excel .... Equis sells a SDK as well, but it's not too cheap (read a couple of hundreds, if I rememeber well) and much more difficult to use.

Other than that, if you search hard enough, you'll find some format descriptions that were hacked together by bit fiddlers on the net.